In general, a semiconductor light emitting device, for example, a light emitting diode, has a stack structure in which an n-type semiconductor layer, a light emitting region, and a p-type semiconductor layer are stacked on a substrate. An electrode is formed on the p-type semiconductor layer and the n-type semiconductor layer. As an electron injected from the semiconductor layer recombines a hole, light is generated from the light emitting region. The light generated from the light emitting region is emitted from a light-transmittable electrode on the p-type semiconductor layer or from the substrate. Here, the light-transmittable electrode on an almost entire surface of the p-type semiconductor layer is a light-transmittable electrode formed of a thin metal layer or a transparent conductive layer.
The color (i.e., wavelength) of light emitted from the light emitting diode is determined depending on a semiconductor material used to manufacture the light emitting diode. This is because the wavelength of the emitted light corresponds to a band gap of the semiconductor material, which is defined as an energy difference between electrons in the valence band and electrons in the conduction band.
FIG. 1 is an exemplary schematic view showing a structure of a related art semiconductor light emitting device.
As shown in FIG. 1, the related semiconductor light emitting device includes a substrate 11, a buffer layer 12, an n-type cladding layer 13, an active layer 14, a p-type cladding layer 15, a p-type contact layer 16, a p-type electrode 17, and an n-type electrode 18. Because the stack structure of the semiconductor light emitting device is controlled at the atomic level, substrate processing is performed to provide the substrate with flatness of a mirror surface. Thus, the semiconductor layers 12, 13 and 15, the active layer 14, and the electrodes 17 and 18 are stacked on the substrate 11 parallel to one another.
The semiconductor layer has a great refractive index, and a waveguide is constructed by a surface of the p-type cladding layer 15, and a surface of the substrate 11. Accordingly, when light is incident upon a surface of the p-type electrode 17 or a surface of the substrate 11 at a predetermined critical angle or greater, the incident light is reflected by an interface between the p-type electrode 17 and the p-type cladding layer 15, or by the surface of the substrate 11, and propagates through the inside of the stack structure of the semiconductor layers in a horizontal direction. During such propagation, light is confined within the waveguide and may be lost. For this reason, external quantum efficiency cannot be achieved as much as expected.
As one method to eliminate aforementioned defections and improve the external quantum efficiency, a semiconductor light emitting device as illustrated in FIG. 2 has been proposed. FIG. 2 is a schematic view showing a different example of the related art semiconductor light emitting device.
As shown in FIG. 2, the improved semiconductor light emitting device includes a substrate 21, a buffer layer 22, an n-type cladding layer 23, an active layer 24, an p-type cladding layer 25, a p-type contact layer 26, a p-type electrode 27, and an n-type electrode 28.
As compared to the related art semiconductor light emitting device illustrated in FIG. 1, the substrate 21 of the improved semiconductor light emitting device has a top surface with uneven patterns. The buffer layer 22 is formed on the uneven surface of the substrate 21, and the n-type cladding layer 23 is stacked thereon.
In the semiconductor light emitting device having the flat substrate illustrated in FIG. 1, when light propagates in the semiconductor layer in the horizontal direction, a portion of the light is absorbed into the semiconductor layer or the electrode, which causes attenuation of light coming out of the semiconductor layer.
In comparison, as for the improved semiconductor light emitting device illustrated in FIG. 2, light propagating in the horizontal direction is scattered or diffracted. Thus, light can be efficiently emitted from the upper semiconductor layers 22, 23, 24 and 25 or from the lower substrate 21, contributing to improving the external quantum efficiency.
In the case of the improved semiconductor light emitting device illustrated in FIG. 2, injected carriers move on surfaces and interfaces, thereby generating electrical conductivity. A high drive voltage applied within a predetermined area for the light emission allows a large amount of injected carriers (electrons) to flow. Here, a current flows through a thin layer of the n-type cladding layer 23 located under the n-type electrode 28. A current density-concentrated region is located closely to the n-type electrode 28.
In this region, current crowding occurs, which makes the current density relatively high. When such a region where the current distribution is concentrated is generated, a temperature drastically increases. Then, generated heat lowers the light emitting efficiency, and consequently, reliability of a device is reduced in the long run.
Also, the proximity of the two electrodes makes the semiconductor light emitting device vulnerable to static electricity. Because the substrate is an insulating substrate, thermal dissipation is not improved even if a heat sink is attached to the substrate.
Therefore, research is made on a semiconductor light emitting device that can improve the external quantum efficiency and the thermal dissipation.